From the beginning of the jet age, safety regulators wanted an assurance that if an airliner had an engine failure over water, it could still make it back to land—and that meant having at least two functioning engines left.

As a result, all long distance flying between continents was done on planes like the four-engined Boeing 747, the four-engined Airbus A340, or the three-engined McDonnell Douglas MD-11.

How different it is now. Engine failure is now such a rare event and engines are so much more powerful that an airliner left with only one engine is deemed safe enough to fly on routes where it can be more than five hours flying time from the nearest airport.

There have been no instances of a twin-engined jet, having lost power, having to ditch in the ocean. But there has been one white-knuckle ride for the passengers of a United Airlines 777 flying nonstop from New Zealand to Los Angeles. The plane lost the power of one engine beyond the halfway point and diverted to Hawaii’s Big Island. Against strong headwinds, it took 192 minutes to reach the runway.

The 777 is the poster child for this revolution—it’s the airlines’ favorite for the longest routes over water. With 1,500 777s delivered, nobody had died in one until the crash of Asiana Flight 214 in San Francisco last summer—and that was due to the pilots mishandling a landing, not an engine failure.

Ironically, with this new version of the 777, Boeing will eventually be killing off the demand for its legendary 747, the truly transforming and much loved machine that was the first airplane to bring overseas travel to the world’s masses in the 1970s.

“The greater efficiencies of the two-engined 777 mean that it is able to fly nearly as many passengers, around 400, more cheaply.”

The greater efficiencies (principally markedly less fuel consumption) of the two-engined 777 over the four-engined 747 mean that it is able to fly nearly as many passengers, around 400, more cheaply. As far as the 450-passenger 747 goes, its latest version, the 747-8, is selling far more strongly as an all-cargo carrier than as a passenger jet.

But by Boeing’s reasoning it’s better to replace your own icon than let someone else do it—and that someone else is Airbus. Airbus was forced to stop producing its four-engined A340 because it became too costly for airlines to operate it and remain competitive. The two-engined A330, meanwhile, has thrived.

Last year Airbus produced a direct competitor to the 777, its two-engined A350, bigger and more efficient than the A330. This reflected what all the airlines now saw as the optimal-sized plane for intercontinental routes between hundreds of cities, called point-to-point.

Boeing has retaliated by giving the 777 new engines and a new lighter, advanced-technology wing built of composites, not metal, that they claim will outperform the A350. That is why Boeing’s deal with the machinists’ union is so crucial to both the company and the union: the 777 has a huge future market that will last decades. With the smaller 787 Dreamliner, also aimed at the point-to-point routes, Boeing believes that the two models give it an edge over Airbus.

All this leaves just one four-engined jet with a future, albeit a limited one: the Airbus A380 superjumbo that can carry between 500 and 700 passengers.

The A380 is not sized for all those point-to-point routes, but for long distance flights between significantly fewer major world hubs. Given the enormous growth expected in point-to-point travel, the market for the A380 is limited (one airline, Emirates, has ordered a third of the 259 A350s that have been sold while building a mammoth new hub in Dubai), and it’s unlikely that the A380 will ever turn a profit for Airbus.

The A380 needs four engines not for safety reasons but just to lift its massive loads. And, as it turned out, having four engines was crucial in the most serious A380 emergency yet when a Qantas A380 suffered an engine explosion on its climb out of Singapore’s Changi airport in November 2010. The plane, badly damaged, was able to return to Singapore on the power of its remaining three engines, even though all of those were degraded. Even then, it was a close-call for the 469 people aboard.